1. Field of the Invention
The present invention relates to a method of manufacturing optical glass having a gradient index and, more particularly, a method of manufacturing optical glass having a gradient index in a direction of the optical axis.
2. Prior Art
Optical glass is generally required to have a uniform refractive index. Optical glass and lens materials having gradient indices having controlled profiles have been developed and commercially available in recent years.
Gradient index lenses are classified into the following three types of lens:
Radial gradient index lens;
Axial gradient index lens; and
Spherical gradient index lens.
An example of the axial gradient index lens is shown in FIG. 19. A lens 1 has a gradient index monotonously decreased from a vertex 1A of its convex spherical surface to almost perfectly correct spherical aberration occurring on the convex spherical surface. A function for the axial gradient index can greatly enhance the aberrational correction by defining a refractive index n(z) at a distance z, as shown in FIG. 19, as follows: EQU n(z)=n0-kz (1)
where n0 is the refractive index at the vertex 1A of the convex surface, z is the length in the direction of thickness along the optical axis when the vertex of the convex surface is defined as an origin, and k is a positive constant. The distribution defined by equation (1) is called a linear distribution hereinafter. In order to obtain a lens having excellent optical characteristics, axial gradient index glass having a linear distribution or a distribution close to a linear distribution is prepared, and the glass is formed into a lens. When a refractive index difference .DELTA.n shown in FIG. 19 is increased, freedom in lens design is increased to facilitate aberrational correction. Therefore, a larger refractive index difference .DELTA.n is preferred.
An ion exchange method is generally employed as a method of manufacturing axial gradient index glass. More specifically, a flat glass body containing a monovalent ion component is dipped in a molten salt such as KNO.sub.3, NaNO.sub.3 or TlNO.sub.3 to form a concentration distribution of Li+, Na+, K+ or Tl+ ions in the glass body, thereby obtaining a gradient index corresponding to the concentration distribution.
Since, however, a monovalent ion exchange is a diffusion phenomenon, a gradient index obtained by the ion exchange method is not linear as shown in FIG. 19, but has one of the following distribution curves as shown in FIG. 20:
Distribution curve having an upward convex shape;
Distribution curve having a downward convex shape; and
S-shaped distribution curve.
Even if a glass material having such a gradient index is formed into a lens, it is difficult to correct the aberration.